The present invention relates in general to communication systems, and is particularly directed to a new and improved, closed loop, Active Cancellation Technique (ACT)-based RF power amplifier linearization architecture having parallel RF amplifiers coupled in intermod-complementing predistortion paths of the type disclosed in the U.S. Patent to Mucinieks, U.S. Pat. No. 6,111,462, (hereinafter referred to as the ""462 patent and the disclosure of which is incorporated herein). By injecting a pilot tone into the signal transport paths of each of the RF power amplifiers, the invention is able to close a set of vector modulation control loops and thereby track and cancel intermodulation distortion products from the composite output of the pair of RF amplifiers.
Communication service providers are subject to very strict bandwidth usage spectrum constraints, including technically mandated specifications and regulations imposed by the Federal Communications Commission (FCC). These rules require that sideband spillage, namely the amount of energy spillover outside a licensed band of interest, be sharply attenuated (e.g., on the order of 50 dB). Although these regulations may be easily met for traditional forms of modulation, such as FM, they are difficult to achieve using more contemporary, digitally based modulation formats, such as M-ary modulation.
Attenuating the sidebands sufficiently to meet industry and regulatory-based requirements by such modulation techniques requires very linear signal processing systems and components. Although linear components can be produced at a reasonable cost at the relatively narrow bandwidths (baseband) of telephone networks, linearizing inherently non-linear components such as RF power amplifiers can be prohibitively expensive.
A fundamental difficulty in linearizing RF power amplifiers is the fact that they generate unwanted intermodulation distortion products (IMDs) which manifest themselves as spurious signals in the amplified RF output signal, such as spectral regrowth or spreading of a compact spectrum into spectral regions that do not appear in the RF input signal. This distortion causes the phase/amplitude of the amplified output signal to depart from the phase/amplitude of the input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the RF input signal.
A brute force and relatively inefficient approach to linearize an RF power amplifier is to build the RF amplifier as a large, high power device, and then operate the amplifier at a very low power level (namely, at only a small percentage of its rated output power), where the RF amplifier""s transfer characteristic is substantially linear. An obvious drawback to this approach is the overkill penaltyxe2x80x94a costly, inefficient and large sized RF device.
Other prior art linearization techniques include baseband polar (or Cartesian) feedback, post-amplification, feed-forward correction, and pre-amplification, pre-distortion correction. In the first approach, the output of the RF power amplifier is compared to the input, and a baseband error signal is used to directly modulate the signal which enters the amplifier. In the second approach, error (distortion) present in the RF amplifier""s output signal is extracted, amplified to the proper level, and then reinjected (as a complement of the error signal back) into the output path of the amplifier, so that (ideally) the RF amplifier""s distortion is effectively canceled.
Pursuant to a third approach, a predistortion signal is injected into the RF input signal path upstream of the RF amplifier. Ideally, the predistortion signal has a characteristic equal and opposite to the distortion expected at the output of the RF amplifier. As a result, when subjected to the (distorting) transfer characteristic of the RF amplifier, it effectively cancels the distortion in the output. Predistortion may be made adaptive by measuring the distortion at the output of the RF amplifier and adjusting the predistortion control signal to minimize the distortion of the output signal of the power amplifier during real time operation.
In accordance with the xe2x80x98Active Cancellation Techniquexe2x80x99 (ACT) RF power amplifier linearization scheme described in the ""462 Patent, and shown diagrammatically in FIG. 1, high efficiency RF power amplifier linearization is achieved by an open loop technique that adjusts signal components driving a pair of effectively xe2x80x98matchedxe2x80x99 RF power amplifiers A1 and A2, such that one RF power amplifier xe2x80x98pre-distortsxe2x80x99 the other. Being matched implies that the two amplifiers A1, A2 have essentially the same transfer characteristicsxe2x80x94both in terms of their intended RF performance and unwanted IMD components they inherently introduce into their amplified outputs.
More particularly, an RF input signal to be amplified is split by a directional coupler CPL1 into two paths. A first path includes an attenuator or scaling pad ATT and a controlled gain adjustment G1 and a phase adjustment element "PHgr"1, which adjust the amplitude and phase of the RF input signal prior to being amplified by the main amplifier A1. The output of the main path amplifier A1 is coupled through a delay stage DL2 to a first input of an output combining stage OCS (such as a quadrature hybrid).
A second split RF input signal path is used to derive a signal containing both the original RF input signal to be amplified by the second xe2x80x98matchedxe2x80x99 amplifier A2, and a complementary version of the IMD products which each of the two amplifiers inherently introduces. IMD products are extracted using carrier cancellation circuitry WS1-WC1 similar to that found in most conventional feed-forward RF power amplifiers. The extracted distortion products are adjusted in amplitude and phase by gain and phase control elements G1 and "PHgr"1 and combined with an appropriately delayed and scaled sample of the RF input signal at WC2.
For this purpose, the second path from the directional coupler CPL1 is coupled through a delay stage DL1 to a first input of (Wilkinson) splitter WS1, a first output of which is coupled to (Wilkinson) combiner WC1. A second output of splitter WS1 is coupled through a variable gain stage G2 to a first input of further (Wilkinson) combiner WC2, a second input of which is coupled to the output of the combiner WC1. A second input of combiner WC1 is coupled to a directional coupler CPL2 installed in the output path of main path amplifier A1.
The output of combiner WC2, which is a composite of the RF input signal and complementary distortion products extracted from the RF amplifier A1, is coupled through a variable gain stage G3 and variable phase adjustor "PHgr"2 to the matched RF amplifier A2. The output of RF amplifier A2 is coupled to a second input of output combining stage OCS.
The amplitude of the RF input signal component of the composite RF signal driving the amplifier A2 is adjusted to be the same as the amplitude of the pure RF input signal driving amplifier A1. Namely, the phase and amplitude of the distortion products are adjusted so that they not only cancel the distortion products generated by the input signals applied to the error amplifier A2, but also replace these distortion products with equal amplitude anti-phase replicas of these products. Thus, the delayed output of amplifier A1 and the undelayed output of the amplifier A2 contain equal phase and amplitude amplified RF input signals and equal amplitude anti-phase distortion products. Thus, distortion components resulting from the RF input signal components driving both amplifiers are essentially the same.
In the output combining stage OCS, these signals are summed, so that (desired) amplified RF (carrier) signals add and (unwanted) distortion products cancel. The output from the combining stage OCS is therefore an amplified version of the RF input signal, that is substantially free of distortion, even though both amplifiers contain distortion products at their outputs. Both amplifiers contribute essentially equal amounts of amplification power to the output of the overall system. Operating efficiency is better than that of a conventional feed forward amplifier because essentially the entirety of both amplifiers"" output power appears at the output of the combining stage.
It should be noted that the ACT architecture of FIG. 1 is not a classic feed-forward architecture. Rather, it is a very effective type of dual amplifier-based, RF pre-distortion amplifier structure, in which the source of the energy used to pre-distort the matched amplifier A2 is produced by an identical (main) amplifier A1, driven by essentially the same input signals as its matched counterpart. The level of distortion components in the energy driving the matched amplifier A2 is on the order of 30 dB below the RF input signal component. Thus, the dynamics of both amplifiers is controlled by the dominant input signal energy.
Now although the ACT amplifier-based linearization scheme described in the ""462 Patent is very effective for achieving a level of non-linear distortion correction at least on the order of 20 dB and greater, a given production device may not be capable of maintaining this level of performance over a wide range of ambient temperature and varying power supply voltage.
One of the reasons for this potential performance shortcoming of the linearization scheme of the ""462 patent is the fact that it is an open loop architecture, and operates on the assumption that since the attenuators which adjust the power to the xe2x80x98matchedxe2x80x99 main and power amplifiers are slaved together, it can be reasonably inferred that the resulting output signal and distortion energy delivered by each amplifier will be exactly the same. However, investigation by the present inventors on substantial numbers of practical production power amplifiers linearized in accordance with the ""462 Patent approach has shown this not to be the case.
Pursuant to a first embodiment of the invention, this amplifier output signal and distortion energy inequality problem is effectively remedied by injecting pilot tone as a xe2x80x98pseudo distortionxe2x80x99 signal into the signal transport paths of each of the pair of RF power amplifiers, in order to track intermodulation distortion products produced by each amplifier. Prescribed signal transport paths of the dual amplifier architecture are monitored by a set of minimization control loops, which control associated vector modulators, such that both the injected pilot tones and intermodulation distortion products are canceled, while RF carrier components are mutually reinforced or constructively sum in the composite output of the pair of RF amplifiers.
To this end, the signal flow path to one of the two xe2x80x98matchedxe2x80x99 RF amplifiers includes a first vector modulator, which is controlled by a first digital signal processor-executed carrier power control mechanism. This first carrier power control mechanism monitors carrier power measured by a detector at the output of the carrier cancellation loop, and adjusts the operation of the first vector modulator, so as to effectively minimize carrier energy, leaving only an injected pilot tone and amplifier distortion energy at the output of the carrier cancellation loop.
A relatively low level, out-of-band pilot tone, which serves as a xe2x80x98pseudo distortionxe2x80x99 signal, is injected into the signal transport paths of the RF amplifiers, and is used to track and cancel intermodulation distortion products produced by each amplifier. The amplifier outputs thus contain a pilot tone component in addition to the desired RF signal and undesired IMDs. Since neither IMDs nor the pilot tone are part of the desired modulated carrier signal being amplified, they constitute unwanted distortion. The use of the pilot tone as a xe2x80x98pseudo distortionxe2x80x99 signal allows the pilot to be treated as representative of whatever noise or distortion is produced by the amplifier pair. By minimizing the contribution of the pilot tone to the composite output signal produced by the amplifier pair, IMDs are also minimized.
A further vector modulator is also installed in the first signal transport path feeding the second amplifier, and is controlled in accordance with a monitored pilot tone-based control loop, so that the injected pilot tone (and therefore any intermodulation distortion products) cancel, at the composite output of the two RF amplifiers.
A second vector modulator is further installed in the second signal transport path feeding the second amplifier, and is controlled in accordance with a power detector coupled to a terminated port of an output combiner. When the desired RF carrier components at the terminated port of the output combiner null, the carrier power levels at first and second input ports of the output combiner are essentially equal and sum constructively at the output signal port of the combiner.
The second vector modulator adjusts a component of input RF carrier energy so as to ensure that the RF carrier energy applied to the second amplifier is the same as that applied to the one amplifier. For this purpose, the output of the power detector monitoring the terminated output of the output combiner is applied to a second carrier power minimization-based control mechanism within the digital signal processor. This second carrier power control mechanism controls the second vector modulator so as to minimize any carrier leakage energy at the output combiner""s terminated port, and thereby equalize the carrier inputs to the two amplifiers.
The amplitude and phase of the pilot energy injected into the second amplifier is adjusted by a gain/phase adjustor, so that when distortion is minimized at the output of the composite amplifier, the pilot energy is also minimized. The vector modulator installed in the first signal transport path is controlled so as to minimize both pilot energy and distortion at the signal output port of the output combiner. This ensures that the total contribution of the pilot tone component from the output of the first amplifier is exactly the opposite of the pilot tone component in the output of the second amplifier.
In accordance with a second embodiment, a feed-forward loop is wrapped around the closed loop, pre-distortion architecture of the first embodiment. The addition of the feed forward stage enables the integrated amplifier architecture containing the RF amplifier linearization stage and the feed forward stage to deliver extremely high linearity, including the ability to routinely achieve carrier-to-distortion ratios as high as 85 dB. Efficiency is significantly better than that which is typically seen in competitive dual loop feed forward power amplifiers.